Millimeter-wave signals suffer from increased pathloss, severe channel intermittency, and inability to penetrate through most common materials, thus making the propagation conditions more demanding than at lower frequencies. To overcome these issues, next-generation cellular systems must provide a mechanism by which users and mmWave base stations establish highly directional transmission links to recover a more sustainable communication quality. In this context, directional links require fine alignment of the transmitter and the receiver beams, an operation which might dramatically increase the time it takes to access the network. Moreover, the dynamics of the mmWave channel imply that the directional path to any cell can deteriorate rapidly, necessitating the need for intensive tracking of the mobile terminal.

Therefore, periodical monitoring of the channel quality between each UE-mmWave eNB pair, to perform a variety of control tasks (including handover, path selection, radio link failure detection and recovery, beam adaptation), is fundamental to provide efficient mobility-management schemes.

The tracking of the downlink channel quality is relatively straightforward in 3GPP LTE and is based on the cell reference signal (CRS) that is continuously and omnidirectionally sent from each eNB. However, a CRS will likely not be available in mmWave systems, since downlink transmissions at mmWave frequencies will be directional and specific to the user.

For this reason, in our work, we proposed an uplink measurement system to perform fast beam realignment and/or handover. This scheme was based on multi-connectivity (MC), to benefit from both the high capacities of mmWave channels, as well as from the more robust, but lower capacity, sub-6 GHz links. We demonstrated that the uplink control signaling enables the network to track the angular directions of communication to the UE on all possible links simultaneously, so that, when a paths witch is necessitated, no directional search needs to be performed (this approach greatly saves switch time, since directional scanning dominates the delay in establishing a new link). Additionally, in the case when the mmWave links are not available, the network is able to send the scheduling and serving cell decisions over the LTE cells, since legacy bands are almost transparent to obstacles. Such multi-frequency control signaling can be exploited to implement more robust and stable resource allocation and network management.

We also presented an innovative tracking technique by which the UE can alternate exhaustive scans of the whole angular space (i.e., to determine the optimal surrounding mmWave eNB to connect to, and eventually trigger a handover event accordingly) to more frequent (and faster) refinements operations (i.e., to adapt the beam if the previously optimal configuration has degraded). We argue that the proposed procedure is particularly suited to highly variant and unstable environments, or as a support to the legacy tracking operations when frequent complete angular sweeps are not executable.

Further work is still needed. In particular, due to the lack of temporally correlated mmWave channel measurements, it is currently not possible to develop an accurate analytical model for mobility-related scenarios, which on the other hand remains a very interesting and relevant item for future research.